Launch QEMU with QMP:
-----------------------------------------------------------------------
$> x86_64-softmmu/qemu-system-x86_64 -display none -no-user-config \
    -nodefaults -monitor stdio -machine pc,accel=kvm,usb=off \
    -cpu IvyBridge -m 2048 
    -blockdev node-name=node-Base,driver=qcow2,file.driver=file,file.filename=./base.qcow2 \
    -device virtio-blk,drive=node-Base,id=virtio0 \
    -qmp unix:./qmp-sock,server,nowait
-----------------------------------------------------------------------

Launch `qmp-shell`:
-----------------------------------------------------------------------
$> ./qmp-shell -v -p /export/qmp-sock
(QEMU) query-version
{
    "execute": "query-version", 
    "arguments": {}
}
{
    "return": {
        "qemu": {
            "micro": 50, 
            "major": 2, 
            "minor": 12
        }, 
        "package": "v2.12.0-1690-g81e21aa70c-dirty"
    }
}
(QEMU) 
-----------------------------------------------------------------------

Run `blockdev-create`: On the "file" laer, create an empty file of size
'0', without any image format in it (this makes more sense for network
protocols):
-----------------------------------------------------------------------
(QEMU) blockdev-create job-id=job0 options={"driver":"file","filename":"foo.qcow2","size":0}
{                                                                                                                                              
    "execute": "blockdev-create",                                                                                                                  
    "arguments": {                                                                                                                                 
        "job-id": "job0",                                                                                                                          
        "options": {                                                                                                                               
            "size": 0,                                                                                                                               
            "driver": "file",                  
            "filename": "foo.qcow2"            
        }                                      
    }                                          
}
{                                                                                                                                                                                             
    "return": {}                                                                                                                                                                              
}
(QEMU)
{u'timestamp': {u'seconds': 1530607920, u'microseconds': 27852}, u'data': {u'status': u'null', u'id': u'job1'}, u'event': u'JOB_STATUS_CHANGE'}                                               
{u'timestamp': {u'seconds': 1530608045, u'microseconds': 217758}, u'data': {u'status': u'created', u'id': u'job1'}, u'event': u'JOB_STATUS_CHANGE'}                                           
{u'timestamp': {u'seconds': 1530608045, u'microseconds': 217899}, u'data': {u'status': u'running', u'id': u'job1'}, u'event': u'JOB_STATUS_CHANGE'}                                           
{u'timestamp': {u'seconds': 1530608045, u'microseconds': 218513}, u'data': {u'status': u'aborting', u'id': u'job1'}, u'event': u'JOB_STATUS_CHANGE'}
{u'timestamp': {u'seconds': 1530608045, u'microseconds': 218626}, u'data': {u'status': u'concluded', u'id': u'job1'}, u'event': u'JOB_STATUS_CHANGE'}
-----------------------------------------------------------------------

Now re-run `blockdev-create` to create the "qcow2" format on the above
created file:
-----------------------------------------------------------------------
(QEMU) blockdev-create job-id=job1 options={"driver":"qcow2","size":16384,"file":{"driver":"file","filename":"foo.qcow2"}}                                                                    
{                                                                                                                                                                                             
    "execute": "blockdev-create",                                                                                                                                                             
    "arguments": {                                                                                                                                                                            
        "job-id": "job1",                                                                                                                                                                     
        "options": {                                                                                                                                                                          
            "driver": "qcow2",                                                                                                                                                                
            "file": {                                                                                                                                                                         
                "driver": "file",                                                                                                                                                             
                "filename": "foo.qcow2"                                                                                                                                                       
            },                                                                                                                                                                                
            "size": 16384
        }                                                                                                                                                                                     
    }                                                                                                                                                                                         
}                                                                                                                                                                                             
{                                                                                                                                                                                             
    "return": {}                                                                                                                                                                              
}                                                                                                                                                                                             
(QEMU)                                                                                                                                                                                        
{u'timestamp': {u'seconds': 1530609183, u'microseconds': 437398}, u'data': {u'status': u'created', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}
{u'timestamp': {u'seconds': 1530609183, u'microseconds': 437534}, u'data': {u'status': u'running', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}
{u'timestamp': {u'seconds': 1530609183, u'microseconds': 438212}, u'data': {u'status': u'waiting', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}
{u'timestamp': {u'seconds': 1530609183, u'microseconds': 438308}, u'data': {u'status': u'pending', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}
{u'timestamp': {u'seconds': 1530609183, u'microseconds': 438391}, u'data': {u'status': u'concluded', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}
-----------------------------------------------------------------------

Query for the jobs:
-----------------------------------------------------------------------
(QEMU) query-jobs
{
    "execute": "query-jobs", 
    "arguments": {}
}
{
    "return": [
        {
            "status": "concluded", 
            "current-progress": 1, 
            "total-progress": 1, 
            "id": "job1", 
            "type": "create"
        }, 
        {
            "status": "concluded", 
            "current-progress": 1, 
            "total-progress": 1, 
            "id": "job0", 
            "type": "create"
        }
    ]
}
(QEMU)
-----------------------------------------------------------------------

Dismiss both the jobs (they can be dismissed the moment you get
'concluded' event; the only reason it stays around is so you can still
query its result):
-----------------------------------------------------------------------
(QEMU) job-dismiss id=job1
{
    "execute": "job-dismiss", 
    "arguments": {
        "id": "job0"
    }
}
{
    "return": {}
}
(QEMU) 
{u'timestamp': {u'seconds': 1530608959, u'microseconds': 208670}, u'data': {u'status': u'null', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}
(QEMU) job-dismiss id=job1
{
    "execute": "job-dismiss", 
    "arguments": {
        "id": "job1"
    }
}
{
    "return": {}
}
(QEMU) 
{u'timestamp': {u'seconds': 1530608963, u'microseconds': 776448}, u'data': {u'status': u'null', u'id': u'job1'}, u'event': u'JOB_STATUS_CHANGE'}
-----------------------------------------------------------------------

Now add the just-created 'foo.qcow2' to the QEMU and give it a
'node-name':
-----------------------------------------------------------------------
(QEMU) blockdev-add driver=qcow2 node-name=node-Foo file={"driver":"file","filename":"foo.qcow2"}
{
    "execute": "blockdev-add", 
    "arguments": {
        "node-name": "node-Foo", 
        "driver": "qcow2", 
        "file": {
            "driver": "file", 
            "filename": "foo.qcow2"
        }
    }
}
{
    "return": {}
}
(QEMU)
-----------------------------------------------------------------------

Optionally, query for the block devices:
-----------------------------------------------------------------------
(QEMU) query-named-block-nodes
[...]

            "bps_wr": 0,
            "drv": "qcow2",
            "node-name": "node-Foo",
            "bps": 0,
            "iops": 0,
            "write_threshold": 0,
            "file": "foo.qcow2",
            [...]
        }, 
[...]  
            "bps_wr": 0,                       
            "drv": "qcow2",                    
            "node-name": "node-Base",
            "bps": 0,
            "iops": 0,
            "write_threshold": 0,
            "file": "./base.qcow2",
            [...]
        },  

-----------------------------------------------------------------------